Tribenzacepentalene dianion and 4,7-disubstituted tribenzodihydroacepentalene derivatives: Formation, reactions, and structural properties of potential tribenzacepentalene precursors

Article abstract of DOI:10.1021/ja00147a009

Upon treatment of tribenzotriquinacene (4a) (R = H) and its centro-alkyl-substituted derivatives 4b-d (R = CH₃, C₂H₅, CH₂Ph) with the strongly basic mixture of n-BuLi and KO^tPen, 2-fold deprotonation combined with (formal) elimination of RH from positions C(1)-C(10) occurs to generate tribenzacepentalene dianior, 5-K₂ with varying ease. Diamon 5-K₂ can be trapped with various electrophiles to give 4,7-disubstituted tribenzodihydroacepentalenes 6c-f in good yields. Compounds 6a-f contain an extremely out-of-plane bent C(1)=C(10) double bond, as shown by X-ray structure analyses of 6b-d, and therefore possess an increased reactivity. 4,7-Dihydrotribenzacepentalene (6a) obtained upon protonation of 5 at -78 °C dimerizes at higher temperatures (≥0 °C) toward the head-to-head [2 + 2] dimer 22. At elevated temperatures (≥220 °C), 22 is cleaved to regenerate 6a which can be trapped with anthracene and tetracyclone to give the highly condensed Diels-Alder adducts 23 and 24. Likewise, 6a generated from 5 at low temperatures can be trapped with 1,3-diphenylisobenzofuran, yielding 25. X-ray structure analyses of dimer 22 and adduct 23 revealed strongly elongated 1,1,2,2-tetraarylethane C-C bond, which are attributed to through-bond π-σ* coupling, in these rigid frameworks. The 4,7-bis(trimethylstannyl)-tribenzodihydroacepentalene (6f) undergoes clean transmetalation with methyllithium to give pure dilithium tribenzacepentalenediide (5-Li₂.) Crystal structure analysis reveals that the dianionic fragments 5-Li₂ in these solvent-separated ion pairs are considerably curved.